1. Field of the Invention
The present invention relates generally to a catalyst for use in a slurry bed Fischer-Tropsch reactor. More particularly, the present invention relates to a method of making a Fischer-Tropsch catalyst with high activity, selectivity and stability. Still more specifically, the present invention relates to a method of producing a Fischer-Tropsch catalyst via precipitation with the addition of a strong base during or immediately following precipitation.
2. Background of the Invention
Fischer-Tropsch (FT) synthesis represents a catalytic method for the creation of synthetic liquid fuels. The reaction occurs by the metal catalysis of an exothermic reaction of synthesis gas, or syngas, which comprises carbon monoxide and hydrogen. Fischer-Tropsch (FT) technology is utilized to convert synthesis gas to valuable hydrocarbon products. The liquid product of the FT process is generally refined to produce a range of synthetic fuels, lubricants and waxes. Often, the FT process is performed in a slurry bubble column reactor (SBCR). The technology of converting synthesis gas originating from natural gas into valuable primarily liquid hydrocarbon products is referred to as Gas To Liquids (GTL) technology. When coal is the raw material for the syngas, the technology is commonly referred to as Coal-To-Liquids (CTL). Fischer-Tropsch technology is one of several conversion techniques included in the broader GTL/CTL technology.
The primary metals utilized as catalysts for FT conversion are cobalt and iron. Iron is favored due to a significantly lower cost. The quantity of catalyst available for catalysis in the reactor dictates the reaction product synthesized. Large scale Fischer-Tropsch reactors utilize complex systems to maintain nearly static quantities of catalyst within the reactor as a means to produce a constant output of product. Attrition, the degradation of the catalyst structure, is a major hurdle in improving FT reactor efficiency.
The physical integrity of unsupported precipitated iron catalyst suffers during slurry phase Fischer-Tropsch synthesis, degrading product quality (solids and iron content in wax) to such an extent that the run may have to be compromised or terminated. Other impacts may be on the wax upgrading, for example hydrogenation system, which is sensitive to the presence of catalytic metal (i.e. iron) in the feed stock. These negative impacts reduce time online for a reactor and increase costs for filtering product, maintaining the reactor, and overall production.
One of the primary difficulties encountered in using iron-based catalysts for carrying out the FT reaction in a slurry bubble column reactor (SBCR) is, therefore, the breakdown of the initial catalyst particles into very small particles, i.e. less than 5 micrometers (also referred to herein as ‘microns’) in size. Although the small particle size is advantageous for increasing surface area and reaction rate of the catalyst, problems arise in separating the small catalyst particles from the wax slurry medium. Separating the catalyst particles from the wax is necessary since, when operating under the most profitable conditions wherein wax is produced, removal of the wax (along with catalyst) from the reactor is required to maintain a constant height of slurry in the reactor.
Breakage of catalyst structure is mainly attributed to physical and chemical attrition. When the catalyst undergoes activation, the starting material, hematite, is converted to iron carbides which have different structures and density. The induced stresses from the transformation lead to particle breakage. Chemical attrition is associated with such structural changes during chemical transformation within the catalyst. Active phase transition from iron oxide to iron metal to iron carbide causes such chemical attrition. Additionally, if the FT reactor is operated at high temperature, e.g. greater than about 280° C., or at a low molar ratio of hydrogen to carbon monoxide, e.g. less than about 0.7, carbon formation via the Boudouard reaction can pry the particles apart. Mechanical action can cause breakup of the particles due to catalyst particles impinging each other or the reactor walls. Physical attrition is mainly contributed to this rubbing and collision of the catalyst particles, resulting in micron sized ‘fines’ material. Such attrition may lead to degradation of product quality (solids and iron content in the wax product) and other undesirable impacts on the wax hydrogenation system, which is generally sensitive to the presence of iron in the feedstock. Very fine material is difficult to settle in primary wax/catalyst separation units and the presence of ultrafines will complicate secondary filtration systems.
Accordingly, there is a need for a stable catalyst and a method of making same, wherein the catalyst exhibits resistance against breakdown while maintaining or enhancing desirable features of an iron catalyst, including high activity and selectivity toward high molecular weight (e.g. C5+) hydrocarbons. Such a catalyst should preferably also facilitate separation of the catalyst from the reaction product.